The present invention relates to boiler controls and more particularly to automatic controls for restarting a hot boiler with temperature matching between the boiler and turbine.
Generally, a typical power plant while operating under load produces a steam temperature as high as 1000.degree. F. The turbine temperature normally is slightly less with the exact temperature value dependent on the load and the resultant turbine valve throttling loss.
It is important that the turbine temperature not be changed too rapidly since rapid thermal changes stress the turbine and shorten its useful life.
A hot plant restart may occur right after an unplanned plant shutdown trip, or after an overnight or weekend shutdown. During a hot power plant restart, the turbine metal temperature is normally higher than that of the boiler steam because the boiler cools faster than the turbine. This is due both to the quicker cooldown rate of the boiler and the inability of the boiler to produce rated steam temperature at low load.
Conventional hot plant restart practice is to admit cooler steam gradually to the turbine until turbine metal and boiler steam temperatures are matched, and then gradually increase load. The load increase must also be gradual to avoid thermal turbine stresses due to increases in boiler steam temperature with increased load.
Power plants have historically been started this way since fast restarts have not been required. However, a need for faster hot plant restarts has been emerging, and specifications for new power plants now typically require temperature matching capability. With reduced national spinning reserve generating capacity and sharp regional load peaks, it is important that power plants be able to be restarted quickly when necessary. A plant tripped off line must, if there are no major problems, be able to be restarted quickly to assure adequate electrical supply. Further, with base loaded nuclear plants in operation, some fossil fueled power plants are shut down at night or during the weekends when the load drops and quick morning restarts are thus needed. Quicker hot plant restarts also consume less fuel and thus provide an energy conservation benefit.
To achieve a quick hot plant restart, some means is required to raise the boiler steam temperature to match that of the turbine. Unfortunately, the steam temperature is a function of both load and pressure in accordance with standard boiler characteristic curves. Perhaps the worst case example of this problem is a quick restart at full pressure after an accidental trip. The turbine is hot, say 980.degree. F., but the steam temperature may be only 850.degree. F. at low load.
Various boiler manufacturers have been responding to user demands and have been starting to implement various schemes to improve the low load temperature capability of the boiler. All techniques utilize bypass valving. However, little or no effort appears to have been directed to the development of automatic restart controls for the various kinds of bypass valve schemes being proposed.